DE68913345T2 - Fluid bed device. - Google Patents

Description

The present invention relates to a fluidized bed device for forming a hydrocarbon gas.

As is known per se, a fluidized bed boiler continuously supplies the fuel to a fluidized bed chamber and air through a distributor plate into the fluidized bed chamber to burn the fuel, fluidize the fluidizable medium and carry out heat exchange in heating tubes arranged inside the fluidized bed chamber. In this fluidized bed boiler, an installation height of the heating tubes and an amount of the fluidized bed medium supplied are set so that the heating tubes are immersed in the fluidized bed. In such a boiler, the heating tubes are immersed in the fluidized bed, and the boiler is operated in a range in which the total heat transfer coefficient does not decrease even if the air flow rate, which is a characteristic of the heat transfer of the fluidized bed, is decreased. Therefore, even if the amounts of supplied fuel and supplied air are reduced and the heat of combustion of the fuel is lowered, if the boiler load is lowered, the heat transfer coefficient and a heat transfer surface area are not substantially reduced. Therefore, the temperature of the fluidized bed may rapidly decrease and the fluidized bed may become inoperable.

In contrast, if the amount of fuel and air supplied is increased with increasing boiler load, the temperature of the fluidized bed can rise rapidly, thereby causing a disturbance, such as slagging of the fluidized bed medium.

To deal with this, US-A-4,279,207 discloses that as the boiler load increases, the amount of fluidized medium increases and the contact area between the fluidized bed and the heating tubes increases, thereby increasing the amount of heat transferred from the fluidized bed to the heating tubes. It also discloses the outflow of the fluidized bed medium as the load decreases (in particular column 10, lines 54 to 62 and column 11, lines 7 to 14).

US-A-4,499-857 shows in particular in column 4, lines 53 to 60 and in column 6, lines 17 to 19, that the height of the fluidized bed medium is controlled depending on the temperature of the fluidized bed.

Mining Engineering, page 244, right column, lines 12 to 19 and Figure 7, published in USA, April 1986, shows how the height of the fluidized bed and the number of heating tubes immersed in the fluidized bed are changed depending on the load.

Another type of improved fluidized bed boiler is further disclosed in US-A-4,768,468.

For example, in the fluidized bed reforming furnace equipped as a fluidized bed combustion device, carrying out a reforming process includes the steps of burning the fuel gas oil in the fluidized bed to transfer the amount of heat developed therein to catalyst tubes and adding the water vapor to the hydrocarbon gas in the catalyst tubes.

If the hydrocarbon gas is methane, the steam reforming reaction can be expressed as:

CH4; + H₂O -T CO - 3H₂

When a height of the fluidized bed of the fluidized bed reforming furnace is as small as 400 to 500 mm and when a temperature of the fluidized bed is set at, for example, 800ºC or about that value suitable for carrying out the reforming process, the fuel gas near a fluidized bed surface (upper surface) is vigorously burned while the gas fuel in an upper space is partially burned. As a result, the temperature of a burned exhaust gas rises up to, for example, 900ºC. An amount of heat corresponding to 100ºC as a difference between 900ºC and the temperature, for example, 800ºC, of the fluidized bed is not utilized for the reforming action but is released from the reforming furnace to the outside, resulting in a heat loss.

The fluidized bed boiler also presents the same phenomenon as above. Namely, even in a case where the fuel to be burned in the fluidized bed is coal, as well as a gas, the fine powder of the same tends to be burned near the fluidized bed surface or in the upper space.

On the other hand, this type of combustion in the fluidized bed brings with it such a phenomenon that a part of the fluidized bed medium is scattered out of the furnace together with the burned exhaust gas. A height of a free The wall unit must be made sufficiently coarse to prevent this phenomenon. This arrangement, however, causes an increase in the size of the device.

A technique known per se (e.g., Saving of Energy, pages 39 to 45, volume 35, No. 13, 1983, published by The Energy Saving Centre Foundation) is explained here, in which a foamed ceramic material (porous ceramic material) provided at a gas outlet formed in the heating furnace equipped with a gas burner serves to absorb the heat emitted from the exhaust gas, and the heat radiated from the furnace surface having a high temperature is used to heat the substance to be heated.

However, it is an unknown manner how the above-mentioned porous ceramic material is attached to the fluidized bed combustion apparatus of the present invention, which apparatus exhibits such characteristics that the fluidized bed medium is capable of being scattered to the outside of the furnace together with the burned gas, or the fuel such as gas, coal and the like is partially burned outside the furnace.

However, there arises a problem inherent in the fluidized bed combustion device for burning the fuel by fluidizing the fluidizable medium, whereby a part of the medium leaks into the upper space and adheres to the holes of the porous ceramic material, thereby causing a load which in turn creates an obstacle to the outflow of the burned gas.

Based on the prior art, the fluidized bed reforming furnace includes a plurality of catalyst tubes arranged in the vertical direction. Lower parts of the tubes are embedded in a fluidized bed into which the fuel and air are fed for combustion. The fluidized bed heated by this combustion is brought into contact with the catalyst tubes, thereby heating the tubes. The water vapor is added to the hydrocarbon fuel fed into the catalyst tubes, and the hydrocarbon fuel undergoes the reforming process by heating.

In the reforming furnace provided with the vertically arranged catalyst tubes, the fluidized bed grows in height enough to allow the embedding of the entire catalyst tubes (from the upper to the lower ends), these tubes extending in the vertical direction. For this reason, a stationary bed height (a height of the fluidized bed medium when it is not fluidized) is about half the length of the catalyst tube. The relatively high fluidized bed causes a noticeable increase in its pressure difference, resulting in a gross loss of performance of a blower provided for the purpose of feeding the air into the fluidized bed. Besides, the fluidized bed medium is scattered high as the height of the fluidized bed increases. It is therefore necessary that the free wall be made high. Thus, the height of the furnace body also increases.

FR-A-2 531 944 discloses a fuel reforming apparatus comprising a number of vertically arranged catalyst-filled filled tubes in a fluidized bed, which tubes can be heated to a high temperature by external heating so as to reform fuel into a gas containing hydrogen as the main component.

The conventional fluidized bed reforming furnace in which the catalyst tubes are arranged in the vertical direction has a feature peculiar to the fluidized bed in that it is possible to keep an internal furnace temperature constant. Additional advantages are that a temperature distribution of the catalyst tubes can be reduced to impart uniform heat transfer and a speed at which a response to load changes takes place can be increased. On the other hand, the height of the fluidized bed is not linearly proportional to a surface velocity. In a region where, for example, the surface velocity is relatively small, the height of the fluidized bed increases considerably with a larger surface velocity. On the other hand, in a region where the surface velocity is relatively large, even if the surface velocity increases, the height of the fluidized bed does not increase appreciably. Therefore, if the surface velocity is reduced to half, a ratio at which the fluidizing media contact the vertically arranged catalyst tubes is not reduced to half. As a result, when the superficial velocity decreases, a ratio of the amount of fuel taken out of the fluidized bed to the amount of heat evolved from the fuel increases, while the temperature of the fluidized bed decreases. As a result, a predetermined reforming process cannot be carried out according to the load of the reforming furnace.

The present invention provides a fluidized bed apparatus for reforming a hydrocarbon gas, comprising a furnace body, at the upper portion of which a burnt gas outlet is formed, a distributor arranged horizontally in the furnace body for dividing the interior of the furnace body into an upper fluidized bed chamber and a lower chamber, and heat transfer tubes loaded with catalysts, characterized in that the tubes are arranged horizontally in the fluidized bed chamber.

According to one aspect of the invention, a fluidized bed combustion apparatus may comprise a gas dispersion (distribution) plate disposed in a lower part of a fluidized bed chamber and a fluidized bed formed by fluidizing the fluidizable medium on the gas dispersion (distribution) plate so that a fuel can be burned in the fluidized bed, characterized by a filter disposed in a burned gas outlet or a free wall unit of the fluidized bed chamber so as to traverse the free wall unit or the gas outlet.

The filter may preferably include the use of wire gauze or a porous ceramic plate.

In this fluidized bed combustion device, the exhaust gas which has been burned in the fluidized bed escapes through the free wall, which is arranged higher than the fluidized bed, into the outlet of the fluidized bed chamber. The exhaust gas passes through the filter and is discharged into an outlet line, at which time the exhaust gas imparts the heat to the filter, which in turn increases in temperature. The filter can then radiate the heat. The radiated heat is then absorbed by the fluidized bed medium in preparation for effective use in the reforming action of the reforming furnace or steam generation of the boiler.

The filter performs a function of preventing the fluidized bed medium from being thrown out of the outlet, thus stably carrying out combustion in the fluidized bed without causing changes in the amount of the fluidized bed medium.

According to another aspect of the invention, a fluidized bed combustion chamber may comprise a gas dispersion plate arranged in a lower part of a fluidized bed chamber, and a fluidized bed arranged by fluidizing the fluidizable medium on the gas dispersion plate, whereby a fuel is burned in the fluidized bed, characterized in that a filter is arranged in a burned gas outlet or on a free wall space of the fluidized bed chamber so as to cross the free wall space or the gas outlet; that a burned gas outlet line provided downstream in the flow direction of the burned gas with respect to the portion at which the filter is arranged is divided into a plurality of sub-lines in a direction transverse to the line; and that each of these compartments so separated is equipped with a valve and a gas injection nozzle arranged between the valve and the filter for the purpose of backwashing the filter.

In this fluidized bed combustion device, the exhaust gas similarly transfers the heat to the filter, and it results that the wire gauze or a porous member re-radiates the heat. The radiated heat is then absorbed by the fluidized bed in preparation for effective use in the reforming action of the reforming furnace or the generation of steam of the boiler.

The filter serves to prevent the scattering of the fluidized bed medium from the outlet so that the combustion in the fluidized bed is carried out with stability and no change is caused in the amount of the fluidized bed medium. During these steps, the valves of the plurality of compartments obtained by compartmentalizing the outlet line installed downstream of the filter are operated. As a result of the valve operation, particles stuck to the filter are eliminated. Specifically, the exhaust gas is discharged into the exhaust line by opening the valve of a certain compartment while the valves of other compartments remain closed. In this state, the backwash gas is ejected from the gas nozzle, whereby the backwash gas impinges on the filter of the section where that valve is positioned and flows from the side opposite to the fluidized bed chamber to a free wall space of the fluidized bed chamber. The medium adhering to the filter is removed so that strain on the filter is avoided. These operations are carried out in turn for each separated compartment and the continuous processes are effected with stability.

According to a further aspect of the invention, a A fluidized bed combustion chamber comprising a gas dispersion plate installed in a lower part of a fluidized bed chamber; and a fluidized bed formed by fluidizing the fluidizable medium on the gas dispersion plate, whereby a fuel is combusted in the fluidized bed, characterized in that a filter is arranged to extend transversely to a free wall or a burned gas outlet, and plates for capturing particles scattered from the fluidized bed are provided below the filter.

Preferably, the plurality of scattered particle trapping plates are arranged substantially in the up and down directions, and passages for the burned gas are formed between the scattered particle trapping plates. Below the trapping plates, there is provided a shutter capable of intermittently controlling a gas flow into the passages or an oscillator for imparting oscillations to the filter.

In this fluidized bed combustion device, the exhaust gas burned in the fluidized bed flows through the free wall space arranged above the fluidized bed into the fluidized bed chamber. In the meantime, the collecting plates for the scattered parts catch a good number of fluidized bed particles and return them to the fluidized bed.

The exhaust gas, which contains a small amount of particles, passes through the scattered particle trapping plates and reaches the filter located in the outlet or a free wall space. The gas is discharged to the eye side of the furnace after passing through the filter. while the particles are captured by the filter.

The exhaust gas gives off heat to the scattered particle trapping plates and the filter as it passes through, and it results in the plates and the filter reaching high temperatures. Then, these components radiate the radiant heat. Following this step, the radiant heat is absorbed by the fluidized medium of the fluidized bed and further used effectively for the reforming action of the reforming furnace or for the generation of boiler steam.

Note that the filter heat is emitted directly into the fluidized medium in the fluidized bed or temporarily transferred to the plates for capturing the scattered particles and from there emitted to the fluidized medium in the fluidized bed.

The scattered particle trapping plates and the filter cooperate to prevent the fluidized medium from scattering from the outlet, thereby achieving stable combustion in the fluidized bed without changing the amount of fluidized medium.

On the basis of this fluidized bed combustion device, the oscillations are imparted to the filter and the particles adhering to the filter are thereby eliminated. In particular, on the occasion of inducing the oscillations, the filter is vibrated in a state in which the gas flows in the part of the filter positioned above the closed gas passages when the oscillation is stopped by closing some gas passages. is achieved by means of the closure, whereby the passages are formed between the plates that capture the scattered particles. In this way, the particles are safely removed from the filter.

As explained above, the oscillations are communicated to the filter in combination with the shutter, and the particles can thereby be released by sequentially closing the combustion gas passages. This allows continuous operations of remarkable stability.

According to a further aspect of the invention, one or both filters and an inner surface of the fluidized bed chamber can be coated with a coating material that has a high emissivity.

Where the filter or the inner wall surface of the furnace of the combustion device is coated with the coating material having a higher radiating ability than that of the filter, the radiating heat can be utilized with higher efficiency, with the result that a temperature of the burned gas at the passage of the ceramic member can be lowered to a level close to the temperature of the fluidized bed.

Note that the filter described above may preferably include, as in the former case, the use of a porous ceramic plate or wire gauze. The porous ceramic plate preferably has a thickness of about 50 mm. The porous ceramic plate has a vacancy percentage of preferably 85 to 90%, a high emissivity (0.75 to 0.8) and a high temperature resistance property. The above-mentioned coating material (preferably, e.g., CHIRANO COAT (trademark), manufactured by Ube Industries, Ltd.) is mainly composed of polymerizate having heat resistance and anti-corrosive properties, which material is formed into ceramic materials at high temperatures of 200 to 700ºC so that it has a high radiance (0.9). The porous ceramic plate has a hole diameter preferably smaller than a particle diameter of the fluidized bed medium (generally 0.3 to 1.0 mm).

The wire mesh is preferably made of stainless steel and its hole diameter is preferably smaller than the particle size of the fluidized bed medium. The hole diameter is typically smaller than 1 mm and in particular 0.3 mm or less, but preferably larger than 0.05 mm and in particular 0.1 mm or more.

According to another embodiment of the present invention, a fluidized bed type reforming furnace may comprise a fluidized bed chamber, catalyst tubes filled with catalysts and provided in the fluidized bed chamber. These tubes supply a hydrocarbon fuel; further, a fluidized bed is provided for heating the catalyst tubes from the outside to convert the hydrocarbon fuel into a gas whose main component is hydrogen. This embodiment is characterized in that the majority of the catalyst tubes are arranged horizontally, and to achieve a horizontal position, the tubes in the fluidized bed chamber are arranged higher than a gas dispersion plate.

The horizontal catalyst tube may include a helical band extending in the tube longitudinal direction along the inner tube peripheral wall, the tube being filled with catalysts having a maximum density. As a preferred catalyst, for example, G-56H-1 provided by Nissan Gardler Corp. or R-67-7H manufactured by Topso Corp., USA is possible. A diameter of the catalyst is preferably 3 to 30 mm, preferably 5 to 20 mm.

The gas dispersion plate described above may consist of two pieces of partition plates arranged with air gaps formed in the vertical direction and a plurality of gas nozzles, the nozzles penetrating the partition plates in the up and down directions. The air gaps may serve as gas fuel supply passages. The gas dispersion plate may be constructed such that the walls of the gas nozzle arranged in the gas fuel supply passage are perforated with small holes through which the gas fuel is introduced.

Based on the arrangement that the plurality of catalyst tubes are placed in the horizontal direction, a set height of each of the catalyst tubes or a height pitch at which the catalyst tubes are arranged correlates with an amount of the fluidized bed medium introduced. The set height and the height pitch can be adjustably set to vary in a ratio at which the fluidized bed medium is brought into contact with the catalyst tubes, that is, the number of tubes embedded in the fluidized bed changes with the height of the fluidized bed according to the load fluctuations. in the reforming furnace. It follows that a total amount of heat is transferred from the fluidized bed to the catalyst tubes in accordance with an increase or decrease in the load (gas amount) of the reforming furnace. A range of fluctuations in the temperature of the fluidized bed can be minimized in this way. It is therefore possible to get rid of, by control, a situation in which the fluidized bed temperature decreases after effecting a good deal of heat exchange as in the case of the arrangement of conventional catalyst tubes in the vertical direction even when the reforming furnace load drops. As a result, the reforming furnace can be stably operated even when the load is small.

The horizontal arrangement of the catalyst tubes allows an extreme reduction in the height of a stationary fluidized bed with respect to the above-mentioned settling height, thus resulting in a so-called flat bed. For this reason, a pressure loss of the fluidized bed gas can be reduced, and a power loss is similarly reduced.

It is also possible to reduce the height to which the fluidized bed medium is spread and the free wall space can also be limited to a small height. This contributes to a reduction in the size of the reforming furnace.

Under such circumstances, the fluidized bed type reforming furnace according to the present invention exhibits the features inherent in the heat transfer of the fluidized bed. The reforming furnace can achieve a uniform Be capable of transferring heat to the catalyst tubes and reforming the hydrocarbon fuel with a high efficiency while maintaining a sufficiently high response speed to load variations.

A helical band may be arranged on the inner wall surface of the catalyst tube loaded with the catalysts with maximum density. When the catalyst tube is arranged horizontally, this arrangement prevents the catalysts from diluting to the greatest extent possible in the upper portion when the catalysts concentrate in the lower portion by their own weight. When a spatial portion is formed in the upper portion, the helical band acts to generate a rotating gas flow passing through the tube and therefore the gas escapes only through the upper gap but not to the downstream side. In this way, it is possible to avoid a situation in which parts of the catalyst abnormally increase in temperature. The helical band can also contribute to further improving the heat transfer coefficient of a loading layer.

The gas fuel is introduced into the plurality of gas nozzles of the gas dispersion plate and then stored into the fluidized bed together with the fluidizing air. At this time, the gas fuel can be uniformly spread in the fluidized bed and burned there to keep the fluidized bed temperature constant. The plurality of catalyst tubes extending along in the lateral directions can be arranged on This way they can be heated evenly.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

Fig. 1 is a vertical sectional view showing an embodiment of the present invention;

Fig. 2 is a vertical sectional view showing another embodiment of the present invention;

Fig. 3 is a sectional view taken substantially along line 3-3 in Figure 2;

Fig. 4 is a sectional view showing another embodiment of the present invention;

Fig. 6 and 7 are vertical sectional views to assist in explaining the function of a closure plate, each showing the inside of the device;

Fig. 8 and 9 are vertical views showing other designs of the closure plate;

Fig. 10 is a vertical sectional view showing another embodiment of the present invention;

Fig. 11 is a sectional view taken substantially along line 11-11 in Figure 10;

Fig. 12 is a vertical sectional view showing a configuration of the inside of the catalyst tube;

Fig. 13 is a sectional view taken substantially along line 13-13 in Figure 12 and

Fig. 14 is a vertical sectional view showing a dispersion plate.

As can be seen from Figure 1, there is shown a vertical sectional view of a fluidized bed reforming furnace which is designed as a fluidized bed combustion device as an illustrative embodiment of the present invention.

This type of reforming furnace makes it possible to produce hydrogen gas by reforming hydrocarbon gas, such as town gas, into steam. The hydrogen gas is fed into a fuel cell (not shown).

A reforming furnace body, generally designated 10, comprises a gas dispersion plate 12, which is placed on the floor provided therein so that it traverses the interior of the furnace. Divided downwards from the gas dispersion plate 12 is a combustion chamber 14, the furnace wall of which is equipped with a gas burner 16. The burner 16 is connected to gas and air supply pipes, not shown.

Above the gas dispersion plate 12, a fluidized bed chamber 17 is formed in which a plurality of catalyst tubes 18 are arranged. The catalyst tubes 18 each occupy a horizontal position and are arranged in several stages in the up and down direction. Note that in this embodiment, the catalyst tubes 18 are arranged vertically in a zigzag configuration in four stages. Each catalyst tube 18 is filled with catalysts for reforming hydrocarbon gas into steam, for example, vanadium or nickel.

The reference symbol F represents a fluidized bed formed by a fluidizable medium, for example sand or aluminum oxide with a small particle diameter. The reference symbol 17a designates a free-wall space formed above the fluidized bed F. An exhaust gas outlet 30 is provided above the fluidized bed chamber 17.

The catalyst tubes 18 are fed with hydrocarbon fuel, for example a town gas, as well as with steam.

The gas dispersion plates 12 are composed of upper partition plates 12a and lower partition plates 12b, which are arranged at intervals in the up and down direction so as to traverse the furnace body 10. Between the upper and lower partition plates 12a and 12b, there are formed spatial portions which respectively form gas fuel supply passages.

Gas nozzles 40 are designed to be stationary so that they cover the upper and lower partition plates 12a and 12b vertically. Gas fuel introduction holes 40a are perforated in the wall surfaces of the gas nozzles 40 corresponding to the gas fuel supply passages 41. The lower ends of the gas nozzles 40 are open and communicate with the combustion chamber 14. Blow-out holes 40b are formed at the upper portions of the gas nozzles 40 for injecting the gas burned in the combustion chamber 14 or the gas fuel introduced from the gas fuel introduction holes 40a into the fluidized bed chamber 17. Cover members 40c are attached to the upper ends of the gas nozzles 40 to prevent fluidized bed medium from entering the blow-out holes 40b. Gas fuel supply openings 41a are formed in the wall of the furnace body 10 and communicate with the gas fuel supply passages 41.

A porous ceramic member 20 is provided at an exhaust gas outlet 30 so as to extend across the outlet. A coating material 21 is applied, as indicated by a broken line in the figure, to the surface of the ceramic member 20 facing the fluidized bed chamber 17 as well as to the inner wall surface of the furnace body, the inner furnace wall surface being positioned higher than the height at which the catalyst tubes 18 are arranged in the fluidized bed chamber 17. The description will next turn to the operation of the fluidized bed reforming furnace thus constructed.

The gas fuel, for example town gas, and the air are fed to the gas burner 16, which in turn burns the gas fuel in the combustion chamber 14. The burned fuel is then discharged from the gas nozzles 40 into the fluidized bed chamber 17. Another gas fuel (for example, an exhaust gas from a fuel battery) is introduced via the fuel inlet port 41a to the gas fuel supply passages 41 formed in the gas dispersion plate 12. The gas fuel is then fed into the fluidized bed chamber 17 via the introduction holes 40a and exhaust holes 40b of the gas nozzles 40.

The gas fuel coming from the combustion chamber 14 and also the gas fuel from the gas fuel supply passages 41 are introduced into the fluidized bed chamber 17, whereby the fluidizable medium charged into the fluidized bed chamber 17 is fluidized and heated to form the fluidized bed F. At the same time, the gas fuel is atomized and burned within the fluidized bed F, and as a result, the fluidized bed F is heated more uniformly.

The catalyst tubes 18 are fed with hydrocarbon fuel gas, for example town gas, and the steam.

The catalyst tubes 18 are heated by the heated fluidized bed F, whereby the hydrocarbon fuel, for example the town gas, flowing through the catalysts, for example vanadium or nickel, which are placed in the catalyst tubes 18, is partially decomposed. The fuel thus decomposed is reformed into a gas composed mainly of hydrogen and carbon monoxide. The reformed gas is then taken out of the catalyst tubes 18 and the resulting hydrogen is fed into the fuel cell.

The exhaust gas burned in the fluidized bed flows from the free wall space 17a to the exhaust gas outlet 30 and passes through the porous ceramic member 20 provided there. The exhaust gas also enters an outlet line 31. During this process, the porous ceramic member 20 is heated by the heat given off by the exhaust gas and the radiant heat is thereby emitted mainly on the upstream side (on the side of the fluidized bed F). The radiant heat is absorbed by the fluidized bed medium, which in turn is heated for the purpose of the reforming process.

The reforming furnace including the porous ceramic member 20 provided at the combustion gas outlet 30 is operated under a condition that a temperature of the fluidized bed F is set at 800°C, and when the burned gas at the inlets of the porous ceramic member 20 reaches 900°C due to the combustion in the free wall space 17a, the temperature of the burned gas at the outlets of the porous ceramic member 20 is up to about 810°C. Where the surface of the porous ceramic member 20 is coated with CHIRANO COAT as a high-emissivity coating material 21, the exhaust gas temperature drops to 800°C and about the temperature of the fluidized bed F. In this case, a difference in a radiation heat quantity Q, which is given by the following formula, is theoretically considered to be in accordance with a difference between a radiant capacity

ε 1 = 0.9 of the CHIRANO COAT used as coating material and a radiation capacity of ε 1 = 0.75 of the porous, ceramic member 20.

where Q is the heat quantity (Kcal/h), t1 is the temperature (ºC) of the wall surface, t2 is the temperature (ºC) of the fluidized bed medium and ε 2 is the radiant capacity of the fluidized bed medium.

On the other hand, during operation, some of the fluidized bed is scattered upward together with the burned exhaust gas and tries to exit from the exhaust gas outlet 30. At this time, the scattered particles are captured by the porous ceramic member 20, thereby preventing them from being scattered. The fluidized bed medium is not discharged from the fluidized bed chamber 17 to the outside of the furnace in any way. There is no loss of the fluidized bed medium. For this reason, a constantly stabilized fluidization can be achieved in the fluidized bed chamber 17, and the combustion also proceeds stably. Besides, a means for capturing the particles separately is not required. The fluidized bed chamber 17 is therefore not increased in configuration, resulting in a reduction in the size of the combustion device.

Figure 2 is a vertical sectional view of a fluidized bed reforming furnace configured as a fluidized bed combustion apparatus in another embodiment of the present invention. Figure 3 is a plan view in the direction of the arrows taken substantially along line 3-3 in Figure 1 and showing an outlet for burned gas. Figure 4 is a sectional view taken substantially along line 4-4 in Figure 3.

A different arrangement with respect to the device shown in Figures 2 to 4 and the device shown in Figure 1 is described below.

In the burnt gas outlet pipe 31, on the downstream side behind the porous ceramic member 20, a dispersion plate 32 is arranged, which divides the cross section of the pipe into two parts so that the dispersion plate extends at a predetermined distance from the surface of the porous ceramic member 20 in such a direction as to follow a gas flow. The pipe 31 of this section is provided with two chambers 33. Each chamber 33 is equipped with a valve 34 which is rotatable about a valve axis 35. The valve axis 35 of each chamber 33 is axially supported on a bearing 36 between an outer wall surface of the outlet pipe 31 and the gas dispersion plate 32. At an axial end of the valve stem 35 on the side of the outer surface of the outlet pipe 31, a drive lever 36 is attached, which is connected to a drive source not shown, for example an air cylinder. The valves 34 of the respective chambers can be opened and closed independently of each other. The valve axes 35 of the two chambers 33 are arranged so that they deviate slightly from each other in the up and down directions.

Between the valve axis 35 and the porous ceramic member 20, the two chambers 33 are provided with gas nozzles 37 for introducing a backwash gas for the porous ceramic member 20. Compressed air pipes (not shown) are connected to the gas nozzles 37.

The other configurations are the same as those shown in Figure 1. Different operations of the device from those shown in Figure 1 will be explained.

A portion of the fluidized bed medium is scattered upwards together with the burnt exhaust gas during operation and makes an attempt to exit from the exhaust gas outlet 30. At this time, the scattered particles are captured by the porous ceramic member 20 provided at the outlet 30. The scattering of the particles is thus prevented. The fluidized bed medium is not discharged from the fluidized bed chamber 17 to the eye side of the furnace. Loss of fluidized bed medium is thus avoided. As explained above, the particles captured by the porous ceramic member 20 are later separated therefrom. The separation of the particles from the ceramic member 20 includes, as shown in the drawings, the following steps: flowing the exhaust gas into the exhaust pipe 31 while one of the valves 34 provided in the two divided chambers 33 remains open (state of the left chamber 33 in Figure 4), then closing the other valve 34, introducing the backwash gas from the gas nozzles 37, substantially sealing a space between the valve 34 and the porous ceramic member 20 (state of the right chamber 33 in Figure 4), and backwashing the porous ceramic member 20 arranged in the closed chamber 33. Note that the arrows shown with broken lines indicate the flow of the backwash gas. The particles thus separated fall down into the fluidized bed F. Next, the valve 34 of the chamber 33 in which the backwash is now carried out is opened, while the valve 34 of the chamber 33 into which the exhaust gas is discharged is closed. The porous ceramic member 20 arranged in this section is subjected to backwashing. Such an alternate backwashing process is carried out periodically or sequentially during operation. The gentle operation is stably carried out without causing loading in the porous ceramic member 20. Of course, during normal operation both valves 34 are open; and the valves need only be opened and closed alternatively during the backwashing process.

The leakage of fluidized bed medium from the fluidized bed chamber 17 can be effectively prevented so that stable fluidization occurs in the chamber. Therefore, the stable combustion can be achieved, and the means for trapping the particles is not separately required. This contributes to a reduction in the configuration of the fluidized bed chamber 17, which contributes to the miniaturization of the combustion apparatus.

Figure 5 is a vertical sectional view showing a fluidized bed reforming furnace designed as a fluidized bed combustion apparatus according to still another embodiment of the present invention. Figures 6 and 7 are enlarged views each showing a main part thereof. A different arrangement of the apparatus shown in Figures 5 and 7 from that shown in Figure 1 will be explained below.

At the outlet 30 there is arranged a transfer pipe 31, which serves to recover the waste heat and is suitable for heating the air or the fuel which is fed into the combustion chamber 14 or the fuel which, as mentioned later, is fed to a feed opening 41.

As shown in Figures 6 and 7, wire gauze 50 is provided above the free wall space 17a and is arranged transversely within the furnace body 10. Below the wire gauze 50, a capture plate 52 is attached for capturing the particles of the fluidized bed medium that are thrown out of the fluidized bed F. In this embodiment, the scattered particle capture plate 52 is arranged so that its upper end is adjacent to the wire gauze 50. Below the scattered particle capture plate 52, a closure plate 54 is provided.

According to this embodiment, the stray particle trapping plate 52 is composed of a hook- and tab-shaped member. The surfaces of the stray particle trapping plate 52 are directed upward and downward. Burnt gas passages 56 (with numerals (1) - (6) in Figures 6 and 7) are formed between the stray particle trapping plates 52 and between the inner wall surface of the furnace body 10 and the stray particle trapping plates 52.

The closure plate 54 is driven in the horizontal direction by a drive unit 58, for example an air cylinder installed outside the furnace body 10. The closure plate 54 is provided with perforation openings 60, each having a width equal to a Distance between the scattered particle capture plates 52, the openings being provided at this distance. The closure plate 54 is arranged, as shown in Figures 6 and 7, to open the passages 56 alternately. In Figure 6, the passage groups 56 (1), 56 (3) and 56 (5) are opened, while the passage groups 56 (2), 56 (4) and 56 (6) are closed. After actuation of the drive unit 58, as shown in Figure 7, the passage groups 56 (2), 56 (4) and 56 (6) which were closed by the closure plate 54 are then opened via the openings 60. In sharp contrast to this, the passage groups 56 (1), 56 (3) and 56 (5) which were opened via the openings 60 are then closed by the closure plate 54.

Of course, in the state of Figure 6, the gas flows only through the passage groups 56 (1), 56 (3) and 56 (5). In contrast, in the state of Figure 7, the gas flows only through the passage groups 56 (2), 56 (4) and 56 (6).

A vibration unit 62, such as a hammer device, is installed outside the furnace body 10 to impart vibrations to the wire gauze 50. The wire gauze 50 is vibrated by the vibration unit 52 so that the particles adhering to the wire gauze are released.

Different modes of operation of the device shown in Figures 5 to 7 compared to those in Figure 1 are described below.

The exhaust gas burned in the fluidized bed F passes through the openings formed in the closure plate 54 and enters the passages 56 between the scattered particle trapping plates 52. The passages 56 each assume a curved configuration to change the direction of the gas flow, with the result that the vast majority of the particles entrained by the gas impinge on the scattered particle trapping plates 52. The particles immediately fall down onto the fluidized bed F. Some of the particles which have penetrated the passages 56 together with the burnt gas adhere to the wire gauze 50, and the remaining particles fly into the gas outlet 30 together with the burnt gas. The burnt gas undergoes heat exchange with the heat transfer tube 31 provided at the outlet 30 and is discharged to the outside of the furnace body 10.

Meanwhile, the wire gauze 50, the stray particle trapping plates 52 and the shutter plate 54 are heated by the heat given off from the exhaust gas, and the radiant heat is given off to the fluidized bed F. The radiant heat is then absorbed by the fluidized bed medium present in the fluidized bed F. The fluidized bed is in turn heated up for use in the reforming process. Note that the heat of the wire gauze 50 is directly communicated to the fluidized bed F or radiated as radiant heat or given off to the stray particle trapping plates 52, and the radiant heat is given off from the stray particle trapping plates 52 to the fluidized bed F.

A certain amount of the heat carried away from the exhaust gas is returned to the fluidized bed F via the wire mesh 50, the scattered particle capture plates 52 and the closure plate 54. As a result, the heat required for the reforming process required energy can be reduced, which leads to a decrease in the amount of fuel consumed.

In the reforming furnace shown in Figures 5 to 7, the device is operated under such a condition that a temperature of the fluidized bed is set at 780°C. A temperature of the burned gas reaches a value of 880°C due to the combustion in the free wall space 17a under the wire gauze 50. In this case, it can be determined that the temperature of the burned gas above the wire gauze 50 is about 790°C and the amount of heat corresponding to this temperature difference is recovered.

The fluidized bed medium is partially scattered upward together with the burned exhaust gas. However, the scattered particles are captured by the scattered particle capture plates 52 as well as by the wire gauze 50. Therefore, the fluidized bed medium from the fluidized bed chamber 17 is not discharged to the outside of the furnace, thus preventing loss of the fluidized bed medium.

In accordance with the present invention, the provision of the scattered particle trapping plates 52 reduces a proportion of particles adhering to the wire gauze 50. However, the particles are gradually adhered to the wire gauze 50 when the fluidized bed combustion apparatus is continuously operated. To release the adhered particles, the oscillation unit 62 is operated to cause the oscillation of the wire gauze 50. When the wire gauze 50 oscillates, the particles adhered to it are detached. In particular, the gas flow is not blown onto the wire gauze 50, the above the passages 56 (56 (2), 56 (4) and 56 (6) in Figure 6) which are in turn closed by the closure plate 54. Therefore, the particles are almost completely detached immediately after the application of the oscillations. It can be seen that the particles detached from the wire mesh 50 and collected on the closure plate 54 subsequently fall from the closure plate 54 onto the fluidized bed F when the closure plate is moved horizontally.

When the closure plate 54 is moved and brought into the state shown in Figure 7, the passage groups 56 (1), 56 (3) and 56 (5) which have remained open are now closed. Instead, the passage groups 56 (2), 56 (4) and 56 (6) remain unclosed. When the wire gauze 50 is subsequently subjected to the oscillations, the particles are almost completely detached from the wire gauze 50 above the opened passage groups 56 (2), 56 (4) and 56 (6).

As explained above, with each movement of the closure plate 54, either the passage groups 56 (1), 56 (3) and 56 (5) or the passage groups 56 (2), 56 (4) and 56 (6) come into the open state, and therefore the burned gas within the fluidized bed chamber 17 escapes upward.

The particles caught on the wire gauze 50 can be safely removed from the wire gauze 50 while the fluidized bed combustion device continues to operate continuously.

In the embodiment described above, the The oscillating unit 62 and the locking device (consisting of the locking plate 54 and the drive unit 58) are provided. However, in accordance with the present invention, the oscillating unit 52 and the locking device may be omitted; or, alternatively, the locking device alone may be omitted.

Figures 8 and 9 are vertical sectional views each showing a different configuration of the scattered particle trapping plates used in the invention. Referring first to Figure 8, there are shown scattered particle trapping plates 52A each curved in a substantially S-shape. Referring to Figure 9, there are shown scattered particle trapping plates 52B each taking the shape of an inverted V. The scattered particle trapping plates 52B of Figure 9 are primarily used in an illustrative embodiment in which the shutter device is not used.

As shown in Figure 9, the scattered particle trapping plates are arranged at the top or bottom based on the multi-stage arrangement. However, the scattered particle trapping plates may also be arranged vertically one above the other in the multi-stages.

All of the above-mentioned illustrative embodiments deal with reforming furnace devices. However, the present invention can also be applied to fluidized bed boilers.

Figure 10 is a vertical sectional view showing a fluidized bed reforming furnace in a further Embodiment of the present invention. Figure 11 is a sectional view taken in the direction of arrows substantially along the line 11-11 of Figure 10. Figure 12 is an enlarged vertical sectional view showing a part X. Figure 13 is a view taken in the direction of arrows substantially along the line 12-12 of Figure 12 for assistance in explaining a step of attaching a helical band. Figure 14 is an enlarged view showing a gas dispersion plate in detail.

A different arrangement of the device in this embodiment compared to that of Figure 1 is explained as follows.

Above the free wall space 17a, a preheater 69 is installed for preheating the gas fuel supplied to the gas burner 16, or the gas fuel introduced into the gas nozzles of a dispersion plate 12, which will be mentioned later, or the hydrocarbon fuel such as town gas using the burnt exhaust gas, the fuel being fed into the catalyst tubes 18. The exhaust gas outlet 30 is formed on the downstream side of the gas from the preheater 69.

As shown in Figure 14, upper and lower partition plates 12a and 12b are coated with adiabatic materials 13. Note that water tubes can be arranged to pass through both the upper and lower partition plates 12a and 12b and the adiabatic materials 13, thus providing a water cooling plate.

As shown in Fig. 11 for assistance in explaining the assembling step, one end of the catalyst tube 18 is fixedly connected to the wall of the furnace body 10; and the other end thereof is freely adjusted. The catalyst tubes 18 are each bent into a U-shape at their free ends and are horizontally arranged at equal intervals so that their heights optionally differ in four steps in the upward and downward directions. Note that reference numeral 10a represents an adiabatic material applied to the inner wall surface of the furnace. Flanges 18a are fixedly connected to the end surfaces of the catalyst tubes 18 which protrude to the outside of the furnace wall. To the flanges 18a is attached a matching flange 18b which is connected to a projecting holding cylinder 18c which in turn receives an adiabatic material 18d and is inserted into the end portion of the catalyst tube 18 to hold the catalysts at maximum density, using bolts and nuts not shown and in a state where a gasket is inserted between the flanges 18a and 18b (see Figure 12). The catalysts 60 or a helical band 70 can be replaced by dismantling the matching flange 18b. On the boiler furnace body 10, as shown in Figure 11, an upper tube 91 of coarse diameter and a lower tube 92 of large diameter are supported at upper and lower inclined positions of the catalyst tube 18 so as to protrude from the furnace body 10. The upper and lower projections of the catalyst tube 18 with respect to the furnace body 10 are connected via tubes 91a and 92a each having a small diameter to the upper and lower tubes 91 and 92 having a large diameter in such a way that thermal expansion is avoided. The hydrocarbon fuel, such as town gas, and the water vapor heated to about 550°C are introduced from the lower large diameter tube 92 through the small diameter tube 92a into the catalyst tube 18. Hydrogen gas and carbon monoxide gas, the temperatures of which, when they receive external heating from the fluidized bed, have reached about 700°C in the catalyst tube 18, are then introduced through the upper small diameter tube 91a of the catalyst tube 18 into the upper large diameter tube 91.

It can be seen from Figures 12 and 13 that the catalyst tube 18 includes a relatively thin helical ribbon 70 made of stainless steel and extending the entire length of the tube 18 along the tube inner wall surface. The variety of catalysts, for example vanadium or nickel, having a solid dimension of about 20φ x 20H are introduced at the highest density into the portion of the catalyst tube 18 positioned in the fluidized bed chamber 17. Into the portion of the catalyst tube 18 projecting from the fluidized bed chamber 17 to the outside of the furnace are introduced filler materials 80 consisting of fragments of stainless steel which have such properties that even the introduction of a good number of fragments maintains a considerable percentage of voids which have little resistance to gas passage. The catalysts 60 introduced into the catalyst tube 18 can be held in a state of maximum density in combination with the filling materials 80 by the holding action of the holding cylinder 18c of the corresponding flange 18b. Incidentally, in the case of filling the catalyst tube 18 with the catalysts 60, the catalysts can be introduced together with the helical band 70, while still achieving maximum tightness.

In the fluidized bed reforming furnace thus constructed, the air and the gas fuel, such as town gas, which are preheated by means of the preheater 69 are supplied to the gas burner 16. Then, the gas fuel is burned in the combustion chamber 14, and the resulting burned gas is introduced into the fluidized bed chamber 17 from the gas nozzles 40. At the same time, a part of the gas fuel is fed into the gas fuel supply passages 41 formed in the gas dispersion plate 12 via the fuel inlet openings 41a, and is further fed into the interior of the fluidized bed chamber 17 from the introduction holes 40a of the gas nozzles 40. After the burned gases are introduced into the fluidized bed chamber 17, the fluidizable medium filling the fluidized bed chamber is fluidized and heated so that the fluidized bed F is formed. At the same time, the gaseous fuel is spread out within the fluidized bed F and burned in it so that the fluidized bed is heated more uniformly. The catalyst tubes 18 are thus heated uniformly.

However, in accordance with this embodiment, when a height of the fluidized bed F varies according to fluctuations in the load of the reforming furnace, the following are adjusted: an adjustment height of the catalyst tubes 18, a vertical distance between the catalyst tubes 18 and an amount of the fluidized bed medium introduced, corresponding to the number of catalyst tubes 18 embedded in the fluidized bed F. When, for example, the load on the reforming furnace reaches its maximum value, the amounts of gas fuels fed to both the gas burner 16 and the gas nozzles 40, and also the amount of air fed there, also reach their maximum values. It follows from this that the height of the fluidized bed F rises to a level A shown in Figure 10, which is sufficient to embed all the catalyst tubes 18 in the fluidized bed F. At an intermediate load, the amounts of gases and air fed in are proportionally reduced and the height of the fluidized bed F drops to a level B or C shown in Figure 10. Catalyst tubes 18 of the uppermost stage or of the two upper stages are exposed to the fluidized bed F. In addition, under the final load, the amounts of gas fuels and air supplied are reduced to minimum values and the height of the fluidized bed F falls to a level D shown in Figure 10. As a result, the catalyst tubes 18 of the uppermost stage or of the two middle stages are exposed to the fluidized bed F, while only the catalyst tubes 18 of the lowest stage are still embedded.

As explained above, when the height of the fluidized bed varies in accordance with fluctuations in the load of the reforming furnace, the number of catalyst tubes 18 embedded in the fluidized bed increases or decreases, thereby increasing or decreasing the surface area of heat transfer. Accordingly, a total amount of heat transferred from the fluidized bed F to the catalyst tubes 18 varies according to the load fluctuations, thereby minimizing a range of fluctuations in the temperature of the fluidized bed F. Even in a conventional case where the load of the reforming furnace decreases, a good part of the heat exchange is completed so that no sharp drop in the temperature of the fluidized bed occurs. Even under the low load, it is feasible to carry out stable operations of the reforming furnace. It is possible to follow the load variations including the low load, and at the same time rapidly. The temperature of the fluidized bed F is kept constant in the range of 800 to 900ºC.

On the other hand, the water vapor and the heated hydrocarbon fuel such as town gas, natural gas or illuminant oil (the description in this embodiment refers to town gas) enter the lower catalyst tubes 18 after flowing from the lower coarse diameter tube 92 to the small diameter tube 92a. The vapor and the fuel smoothly fly through the packing materials 80 and return to the U-shaped curved portion after passing through a layer of catalysts 60. Then, the fuel and the vapor fly through the upper catalyst tubes 18. Meanwhile, the partial decomposition is carried out by uniformly absorbing the heat from the fluidized bed F with the catalysts located on the outside due to the outside catalysis with the fluidized bed F, the catalysts being uniformly heated as a whole as explained earlier. The fuel becomes hydrogen. and a gas having a high concentration of carbon monoxide and then discharged from the upper coarse diameter pipe 91 through the upper small diameter pipe 91a.

A good portion of the hydrogen gas is supplied to the fuel cell. During the reforming process, swirling, spiral or helical flows of the city gases, water vapor or reformed gases are caused along the helical bands 70 in the catalyst tubes 18, these gases being raw materials within the catalysts 60, which are introduced at maximum density. The gases and vapor fly uniformly within the catalysts 60, so that the reforming process takes place with a remarkably high efficiency. Even if gaps are formed in the upper parts of the catalyst tubes 18, because of the spiral flow of the gases, it is feasible to avoid such a phenomenon to the highest possible extent that the gases do not fly through the layers of the lower catalysts 60, but through the upper gaps alone. Therefore, partial damage to the catalysts 18 due to abnormally high temperatures can be avoided, and at the same time, destruction attributable to local heating by catalysts 60 can be suppressed to the utmost, so that the reforming furnace can be stably operated. Each of the catalyst tubes 18 assumes the U-shape and thus exhibits high flexibility with respect to thermal expansion, and even if one end of the catalyst tubes 18 is fixed to the furnace wall, deformation thereof is permitted, thereby preventing damage.

The catalyst tubes 18 are arranged horizontally to stationary height of the fluidized bed F can be reduced considerably. Therefore, the reduction of the power loss as well as the pressure loss during fluidization can be achieved. In addition, the height to which the fluidized bed medium is scattered decreases and the height of the free wall space 17a is thereby reduced.

The burnt gas passing through the fluidized bed F continues to escape through the free wall space 17a. Thereafter, the gas is fed to the preheater 69, where appreciable heat is imparted to the gas fuel and the combustion air in preparation for the thermal exchange for preheating. After this process, the gas is discharged from the outlet 30 to the outside of the furnace.

It is obvious that the configuration of the catalyst tubes 18 is not limited to the U-shape in which the individual tube is curved at its free end, but simple catalyst tubes 18 can also be arranged vertically in multiple stages.

In this embodiment, the catalyst tubes 18 are arranged in four stages in the up and down direction. However, according to the present invention, the catalyst tubes may also be arranged in two, three or more stages.

When the helical band 70 is coated with a ceramic material to exhibit a radiant heat transfer effect associated with far infrared rays, this effect is combined with the effect of generating the helical flow so as to increase the heat transfer coefficient of the input layer. This in turn makes the outer surface of the catalyst tubes 18 even smaller than before.

In the description given above, the chamber arranged under the dispersion plate 12 is equipped with the burner 16. Instead of the burner, however, an air supply unit can also be provided in the chamber. In this case, all the fuels supplied to the fluidized bed can be introduced from an introduction opening 41.

The present invention can provide, firstly, a fluidized bed combustion apparatus for absorbing heat generated by combustion of a fuel in the fluidized bed of a fluidized bed reforming furnace; secondly, a fluidized bed combustion apparatus capable of efficiently recovering a potential heat amount of a combustion gas discharged from the fluidized bed of the fluidized bed combustion apparatus; thirdly, the invention can provide a fluidized bed combustion apparatus capable of performing stable operations while preventing both leakage of the fluidized bed medium by scattering to the outside of the apparatus and loading of a discharge preventing member; and fourthly, the invention can provide a fluidized bed reforming furnace which can receive a gas, the main component of which is hydrogen, from the water vapor and hydrocarbon gas, for example town gas, which gas can also be fed to a combustion cell, for example.

The present invention can provide a fluidized bed combustion apparatus capable of increasing thermal efficiency by reducing an amount of waste heat, preventing scattering of fluidized bed medium and settling thereof, and further enabling smooth operations to be carried out by preventing this settling.

The present invention can also provide a fluidized bed reforming furnace exhibiting such advantages that a height of a fluidized bed can be reduced, pressure and power losses can be reduced, a height of a furnace body is not larger than necessary, temperature variations of the fluidized bed can be reformed with respect to fluctuations in the load of a reforming furnace, and further a predetermined reforming process can be effected in accordance with the load.

Claims (7)

1. A fluidized bed apparatus for reforming a hydrocarbon gas comprising a furnace body (10) formed at its upper portion with an outlet (30) for burned gas; with a distributor (12) arranged horizontally in the furnace body (10) for dividing the interior of the furnace body (10) into an upper fluidized bed chamber (17) and a lower chamber (14); and with heat transfer tubes (18) loaded with catalysts, characterized in that the tubes (18) are arranged horizontally in the fluidized bed chamber (17). 2. A device according to claim 1, wherein the heat transfer tubes (18) comprise two groups of tubes, each group having a different height at which the tubes are arranged horizontally. 3. An apparatus according to claim 1 or 2, further including a filter (20) for capturing particles scattered from the fluidized bed (F), which filter is arranged in the outlet (30) or on a free wall space (17a) in the fluidized bed chamber (17). 4. A device according to claim 3, wherein a lower surface of the filter (20) is coated with a coating material (21) having a higher emissivity than the filter (20). 5. A device according to claim 3 or 4, wherein an inner wall surface of the free wall space (17a) is coated with a coating material (21) that has a higher radiating capacity than the filter (20). 6. A device according to any one of claims 3 to 5, further comprising a means for preventing the particles from adhering to the filter (20). 7. An apparatus according to any one of claims 3 to 6, further comprising trapping plates (52) for trapping particles scattered from the fluidized bed (F), said trapping plates being arranged under the filter (20, 50) at equal intervals with the height of the lower ends of the trapping plates (52) being equalized so that passages (56) for burned gas are formed between the trapping plates (52), each of the gas passages (56) being bent upwards at least once, whereby the particles carried by a gas flow strike the trapping plates (52) and fall downwards; and means (58) for opening and closing the gas passages (56) between the trapping plates (52) with respect to a lower space in the furnace (10), said means (58) including a plate (54) movable horizontally along the lower ends of the trapping plates (52), said plate (54) being in provided with openings (60) at the same distances as the capture plates (52), so that when the plate (54) moves horizontally over a distance corresponding to the distance between the capture plates (52), the gas passages (56) which have remained open are closed and the gas passages (56) which have remained closed are opened.